Thermal inkjet printers operate by expelling a small volume of ink through a plurality of small nozzles in a printhead held in proximity to a medium to be printed upon. The expulsion of droplets of ink from the nozzles relative to a particular position on the medium results in the production of a portion of a desired character or image. Controlled repositioning of the printhead or the medium and another expulsion of ink droplets continues the production of more pixels of the desired character or image. Inks of selected colors may be coupled to individual arrangements of nozzles to produce a multicolored image by the inkjet printer.
Expulsion of an ink droplet in a conventional thermal inkjet printer is a result of rapid thermal heating of the ink to a temperature which exceeds the boiling point of the ink solvent and creates a gas phase bubble of ink. Each nozzle is coupled to a small unique ink firing chamber filled with ink and having an individually addressable heater resistor thermally coupled to the ink. As the bubble nucleates and expands, it displaces a volume of ink which is forced out of the nozzle and deposited on the medium. After deactivation of the heater resistor, the bubble then collapses, and the displaced volume of ink is replenished, by capillary action, from a larger ink reservoir through ink feed channels.
The life of the heater or firing resistors in the thermal inkjet printhead is often limited by cavitation damage caused by the rapidly collapsing drive bubble. In the past, a thick layer of tantalum was used to protect the resistor from the high pressure spikes generated by the collapsing drive bubble. As the tantalum layer thickness increases, more protection is provided so the resistor life generally increases. The disadvantage of this tantalum layer is that it causes an increase in the amount of energy required to eject a drop because the tantalum layer effectively insulates the resistor surface. This increase in firing energy causes the pen to run hotter, which affects print quality and other aspects of reliability.
It is desirable, therefore, to develop a printhead design that increases the life of a firing resistor in a thermal inkjet printhead without incurring the disadvantages of the prior art.